1. Technical Field
The present invention relates to a system and a method for interference detection and identification as well as frequency allocation for wireless systems.
2. Description of Related Art
The license-free 2.4 GHz ISM band is crowded with radio applications. Examples are WLAN, Bluetooth, cordless phones, microwave ovens, etc. Interference to other systems, and vice versa, is a well-known problem in this band.
A system requiring high reliability for its own quality of service, as well as a good co-existence with other systems must choose a set of frequencies that are unused at that particular time, in that particular area. This is known in the art as dynamic/automatic channel allocation, or dynamic/automatic frequency allocation.
The issue of dynamic channel allocation (DCA) is a known problem for over 30 years. DECT is one of the most widespread systems using DCA. DCA is very effective within one particular system.
Adaptive frequency hopping (AFH) is another example of known art. Such systems adapt their allowed frequency table on the basis of prior success to communicate on particular frequencies. In version 1.2, the Bluetooth SIG has adopted AFH.
It is important to prevent the selection of nearby used Wireless Local Area Network (WLAN) frequencies. This is due to the ‘vulnerability’ of WLAN to interference in combination with the widespread success of the technology. WLANs consist of one access point (AP) and associated wireless stations (STA). The AP transmits a beacon as part of its air interface protocol. Without data transmissions taking place, the beacon remains as the only detectable presence in the WLAN, similar to a heartbeat. This is illustrated in FIG. 1, showing the WLAN beacon as a function of time. It should be noted that the interbeacon interval is shown condensed. WLAN deploys such a beacon with a duty cycle of the order of 1%. In most WLAN access points in the market today, a beacon is transmitted about every 100 ms. The length of the beacon packet normally varies around 1 ms, depending on the amount of content actually being transferred. In a practical system, interbeacon arrival times may show some irregularity. A complication is that WLAN uses the listen-before-talk principle (carrier sense multiple access—CSMA, or more generally as Clear Channel Assessment). This means that a WLAN will not transmit when it sees other users at its operating frequency. For this reason, the use of frequencies interfering with a WLAN may completely block the communication between the AP and the STAs of the WLAN.
A known way to detect WLAN access points is the method used by programs such as “Netstumbler”. This method is based on sending a request to access points (APs) and detecting the WLANs based on the replies of the APs. There are two disadvantages to this approach: 1) Some APs have a security feature, by which they will not reply to such requests, unless the request contains the correct Service Set IDentifier (SSID)—thus making them invisible. 2) A device is needed that is capable of transmissions compatible with (parts of) the WLAN PHY/MAC.
It is an object of the invention to provide an improved system and method for interference detection and identification.